Conventional resonators for microwave mainly include waveguide resonators, microstrip resonators, dielectric resonators, and substrate integrated waveguide (SIW) resonators. For these cavities, the tunability is typically achieved by varactors and switches. Conventional cavities for Terahertz and optics are mainly implemented by two facing plane or spherical mirrors. The simplest of these is the plane-parallel or Fabry-Perot (F-P) cavity, consisting of two opposing flat mirrors. For these cavities, tunability is typically achieved by replacing one of the mirrors with autocollimation structures, for example, with autocollimation gratings.